A is incorrect because Mortgageable is an interface, not a class. Both X and Y are classes which implement (realize) the Mortgageable interface.

D is a bit incongruous - this is a class diagram, not an object or deployment diagram.

B and C are the typical features of classes which realize interfaces.

B is a very elegant benefit of using interfaces because it dramatically decreases the coupling in a system. Methods which take a parameter of type Mortgageable only need to know about the interface, and nothing else about the details of the implementing class.